Staphylococcus aureus is well recognized as a virulent pathogen, which continues to cause life-threatening disease. Its invasive potential is clearly demonstrated by its ability to colonize and infect endovascular tissue. Despite its continued frequency and importance, little is known about the pathogenesis of this endovascular infection. Our laboratory has developed an in vitro assay to explore the dynamics of staphylococcal - human endothelial cell interactions. Recently, we have isolated an endothelial cell membrane protein which binds staphylococci. In the present proposal, we plan to explore the role of this protein in the initial binding interaction and the consequences of staphylococcal infection to the endothelial cell. We hypothesize that invasive staphylococcal disease is the result of specific S. aureus adhesin - endothelial cell receptor mechanisms. Injury to the cell and alterations of the cell surface following infection, may help explain some of the devastating clinical complications encountered in patients with this disease. Our specific aims including the following: 1. What are the characteristics of the isolated endothelial cell membrane binding protein? The protein will be further purified and characterized biochemically. Kinetic binding studies with iodinated protein will further elucidate the specificity and affinity of binding to different S. aureus isolates. The protein's effect on S. aureus adherence to endothelial cells in the infection assay will be determined. Polyclonal antibodies, prepared against the purified protein, will be used in immnofluorescent and ultrastructural studies to define the topography of the binding protein on the surface of endothelial cells harvested from different sources. 2. What are the consequences of staphylococcal infection of the endothelial cell? Injury to the cells by staphylococci may result in the modulation of the surface with increased expression of the binding protein, Fc receptors, tissue factor or increase platelet or white blood cell adhesivity which may contribute to the progression of the disease. The influence of infection on these factors will be determined. 3. Are there additional factors for S. aureus on the endothelial cell surface which can be identified? Monoclonal antibodies directed against human endothelial cells will be screened for their ability to block S. aureus adherence to these cells. These antibodies will then be employed to identify, isolate and characterize the receptors.